A common strategy is to engage a salesperson who has worked for similar companies, get them to use their book of contacts to sell your services. If you advertise for the salesperson, you should be able to find someone who will work on a freelance/commission-only basis.
I've heard this, but I've also heard that we should do our own sales. Does anyone have any info on where to find London-based IT sales people that will work on commission only?